It is known to use an energy absorbing interlayer of plasticized polyvinyl butyral (PVB) with one or more rigid layers such as glass in a safety glazing. Such a glazing is usually prepared by interposing the PVB layer between glass sheets while eliminating air from between the engaging surfaces and then subjecting the assembly to elevated temperature and pressure in an autoclave to fusion bond the PVB and glass and form an optically clear structure. These glazings are used in windows such as the front, side and rear windows in motor vehicles, particularly windshields, where the interlayer can absorb a blow from the head of an occupant without penetration of the windshield.
Automobile and homeowners, especially those who have experienced a vehicle break-in, theft or hurricane damage, are increasingly interested in intrusion resistant glazings. Glass has been identified as the weak link in the overall intrusion resistance of vehicles and buildings. For instance, some police reports estimate that broken glass is the entry route for at least 60% of unauthorized entry into passenger vehicles. There is also a belief that a 30-second delay in entry is sufficient to discourage many spontaneous thieves because the increased break-in time and additional noise call attention to the thief. In response, carmakers, at least, are replacing traditional tempered glass with laminated glass for enhanced security. Common automotive laminated glass, though, can be defeated by determined thieves who use more sophisticated tools to puncture by impact and pull laminated glass out from its frame.
Many attempts to improve the performance of glass laminates have been made, including modifying the stiffness and/or impact resistance of the PVB interlayer. For example, U.S. Pat. No. 4,814,529, to Cartier et al., discloses lightly cross-linking PVB resin to selectively increase molecular weight of the PVB and the modulus of a plasticized sheet formed therefrom for use in laminated safety glass assemblies. U.S. Pat. No. 5,246,764, to LaPorte et al., discloses laminated glazing with improved impact strength where mean break height for a dropped mass of a glass laminate increased by dispersing adhesion resistant means on the surface of PVB sheet. U.S. Pat. No. 5,482,767, to Karagiannis et al., discloses glass laminates of improved impact resistance comprising a PVB interlayer having discrete particles of crosslinked polyvinyl butyral integrally randomly dispersed throughout a matrix of PVB.
In recent years additional sophisticated features are appearing in such windows to enhance performance. These include special, layered metal/dielectric stacks for solar radiation control which may also be electrically conductive for defrosting, defogging, etc; holographic layers as solar reflecting mirrors and in head-up displays to facilitate viewing instruments on the vehicle dashboard while looking straight ahead; photochromic and electrochromic layers which controllably change color and/or visible transmission upon exposure to solar radiation or application of a voltage; layered protective antilacerative structures on the inboard side of a conventional three layer glass/PVB sheet/glass laminate to minimize lacerations from sharp edges of broken glass; special plastic layers in bilayer structures replacing one glass layer of such a three layer glass laminate, and similar, functional performance-enhancing layers and coatings. These performance layers are usually deposited on or adhered to a carrier layer which is different from the low modulus, elastomeric PVB which is unsuitable as a carrier. For use in safety glazings a carrier layer should have good clarity, be relatively uniform in thickness and strong having high modulus to facilitate ease of handling and processing during association with the performance layer(s). Frequently biaxially oriented polyethylene terephthalate is used as noted, for example, in U.S. Pat. No. 4,465,736.
Using PET in glass laminates offers many advantages. As U.S. Pat. No. 5,024,895, to Kavanagh et al., and U.S. Pat. No. 5,091,258, to Moran, disclose, PET can be biaxially stretched to improve strength and can be heat stabilized to provide low shrinkage characteristics when subjected to elevated temperatures. The tensile modulus, a desired property of glass laminate interlayers and an indication of the stiffness of the interlayer, 21° C.-25° C. of PET is about 1010 Pa as compared with about 107 Pa for PVB of the type used in safety glazings. This increased stiffness of PET is a desirable property for use in glass laminates.
Many applications of PET involve a layer of PET being used as a carrier for the aforementioned functional layers, such as solar radiation blockers, antennas or heat strips. U.S. Pat. No. 5,979,932, to Jourdaine et al., U.S. Pat. No. 5,091,258, to Moran, and U.S. Pat. No. 5,932,329, to Frost et al., disclose a PET layer between two PVB layers wherein the PET layer is provided with an infrared-reflective coating. U.S. Pat. No. 4,017,661, to Gillery, discloses a composite interlayer wherein a PET sheet is coated with an optically clear, electrically conductive, transparent coating used as a carrier for metal layers which can be electrically heated for defrosting the glass laminate. U.S. Pat. No. 5,024,895, to Kavanagh et al., discloses a PET layer between two PVB layers that includes an integrated infrared-reflective and an electrically conductive coating.
Although most prior art laminates generally provide acceptable resistance to shattering when struck with a blunt object, there is often unacceptable resistance to penetration and pullout. For example, in the case of automobile break-ins, prior art glass laminates often will not shatter, but will break. After being broken, though, the prior art glass laminates usually are significantly weakened and are thus susceptible to being pulled out of their frames. This lack of stiffness in prior art glass laminates can effectively offset the otherwise acceptable shatter-resistant nature of the glass laminates, particularly in automotive break-in situations. Many prior art glass laminates do not exhibit acceptable strength properties after being broken.
Furthermore, although simply increasing the PVB thickness can improve resistance to penetration, this solution does not alleviate the poor resistance to pullout problem. Increased thickness laminates can also be limited by optical clarity. It is of utmost importance that glass laminates used for automotive safety glazings exhibit a high degree of optical clarity; that is, exhibit high level of visible transmission and low levels of optical haze or light scattering. Prior art laminates do not provide a means to achieve a significant increase in laminate stiffness without compromising optical clarity.
It would thus be desirable to develop a glass laminate that exhibited superior penetration resistant qualities, as well as an increased stiffness so as to improve the glass laminate's resistant to pullout after the glass is broken, for example, for use in security glazing applications such as architectural and automotive glazing. Furthermore, it would be desirable to improve the stiffness of a laminate without sacrificing its optical clarity.